For this purpose it is already known to provide a ring of anti-rotation teeth inside the internal cavity at an intermediate level between the head and the skirt. These inwardly projecting teeth have sharp edges between generally flat faces, and it will readily be understood that it is difficult to make such teeth in a piece of glass because of there fragility. Further, the smaller the diameter of the ring and the thinner the glass at said intermediate level, the greater the difficulty.
Preferred embodiments of the present invention avoid these drawbacks.